The subject matter disclosed herein relates to flow regulating devices and, more particularly, to valves that operate under low flow conditions.
Many valves utilize a plug element and a seat element to regulate the flow of fluids (e.g., gas and liquid). When these types of valves are closed, the plug element rests against the seat element. This configuration prevents fluid flow through the valve. In some valves, changes in pressure of the fluid downstream of the valve can lift the plug element off the seat element. The displacement of the plug element causes the valve to open slightly, which forms a small gap between the bottom of the plug element and the top of the seat element. Often, fluid flows through the gap to supply the downstream demand, effectively equalizing the pressure across the valve.
Under certain flow conditions, the gap operates as a nozzle that induces a Venturi nozzle flow. This flow leads to localized changes in the velocity and pressure of the fluid as the fluid exits the nozzle. For example, low flow conditions (e.g., flow of 10% or less of maximum valve capacity) can cause the fluid pressure at the nozzle to drop below the pressure downstream of the valve. The pressure differential causes the valve to close rapidly. As a result of the rapid descent, the plug element may slam violently onto the seat element. In many cases, however, the upstream flow cannot satisfy the downstream demand before the valve closes. The resulting differential allows the valve to open again because the downstream pressure remains below the set-point of the valve. Inevitably, the valve will continue to open and close in a rapid cyclic pattern.
This pattern can generate a loud sound, or “buzz,” that emanates from the valve. For flow devices (e.g., flow regulators) that incorporate these types of valves, the pattern can also induce vibrations that can lead to failure of the valve and, ultimately, damage to the valve, the flow device, and/or components upstream and downstream of the valve. This damage can lead to costly repairs and system downtime.